Touch panel has become a new information input device in recent years. The principle of touch panel is operated by touching the screen. Touch panel is conveniently carrying and human-based function operation. The touch panel can be classified into the following four types according to the induction principle: optical touch panel, acoustical touch panel, capacitive touch panel and resistive touch panel. Recently, the capacitive touch panel has shared the most market. The capacitive touch monitor is constituted by the Indium-Tin-Oxide (ITO) transparent conductive thin film. The capacitive touch monitor determines the movement of finger by the change of capacitance, and the change of capacitance is generated by the ITO electrode when the finger touches the touch panel. Before the finger touches the touch panel, the parasitic capacitance between the transparent electrodes ideally is a fixed capacitance. After the finger touches the touch panel, the finger or the conductive object forms a loop to change the parasitic capacitance between the original transparent electrodes. Thus, while the changing amount of the capacitance can be detected, whether the finger or the conductive object touches the surface of the touch panel can also be determined.
The foldable capacitive touch panel can be disturbed by the condition, such as ambient temperature or humidity. Another obvious noise generated source is the electric signal of the display. Therefore, we have to realize the features of interferences and noises of the touch panel.
For example, using the digital filter can achieve the purpose of decreasing noises. The digital filter uses the charge transfer method to sense the change of capacitance of the touch panel. The charge transfer method contains the effect of resisting the high frequency noise. It is because that the average amplitude of noises is 0. If the charge accumulation cycle is longer, the accumulated amplitude of noises will approach to 0.
However, the most of scanning frequencies of the display is 60 Hz. The noises of this scanning frequency are not all the high frequency noises and may not always contain the phenomenon of the accumulated amplitude of noise approaching to 0. Thus, it may hardly to filter all the noises by the existing IC. This may generate the large amount of false reporting point.
In another aspect, the present technology also provides the method to eliminate the impact of noises by using the differential sensing circuit. The principle of the differential sensing method is that because of the two adjacent sensing electrodes of touch panel will respectively receive the similar amplitude of noises. As such, subtracting the two sensing signals of the two adjacent sensing electrodes will the impact of the noises.
However, the capacitance of the sensing electrodes may be different due to the variance of manufacturing processes. When the finger doesn't touch the touch panel, the capacitance values of the two sensing electrodes are similar and the impacts of noises are also similar. When the finger touches the touch panel, the capacitance values of the two adjacent sensing electrodes are different. But, the impacts of noises are still similar. Thus, by subtracting the two sensing signals of the two adjacent sensing electrodes, it will eliminate the impact of the noises. Besides, only the change of current value caused by the finger will be enhanced by the charge amplifier. Also, the change of current value caused by the finger will be transferred to a voltage signal and then output the voltage signal. However, it is hard to use this method to deal with the signal generated by non-locality stray.